This invention relates to a process for making substantially chromium-free sodium sulfate from a solution thereof which contains 0.2 to 100 ppm Cr(III). (All references herein to chromium concentration are on a 100% Na.sub.2 SO.sub.4 basis.) In particular, it relates to oxidizing Cr(III) in a sodium sulfate solution to Cr(VI) at a pH over 7, evaporating water from the solution to precipitate orthorhombic sodium sulfate, and separating the precipitated orthorhombic sodium sulfate from the solution.
When sodium bichromate is produced, salt cake (sodium sulfate, Na.sub.2 SO.sub.4) is also made. This salt cake contains about 200 to about 1000 ppm Cr(VI). After the chromium has been removed from it the salt cake is useful in manufacturing glass and other products such as detergents. At the present time, chromium is removed from salt cake by reducing it to Cr(III) by adding sulfuric acid and SO.sub.2. Sodium hydroxide is added which results in the precipitation of chromium hydroxide, Cr(OH).sub.3. The chromium hydroxide is separated from the solution and water is evaporated to precipitate the salt cake. However, this purified sodium sulfate still typically contains about 2.0 ppm Cr(III), which is undesirable to glass manufacturers because it imparts a green color to the glass and is undesirable to detergent manufacturers because of the slight green color.